Small single color presses are going to be termed duplicators. All were able to be equiped with a T-Head (developed/manufactured by Townsend Industries, referred to simply as T-Heads) for two color work. Their consistency/abilities is not as full as a true press unit due to there small size and roller train. They are a nice option for line work, small coverage areas. Mind you, many long time operators have done fine work, large image areas & pushed them to the limits. But that is aided by new rollers, all stripes & settings new and set. New rubber is not that expensive, probably far cheaper than fuser/corona/charge units.
If you purchased a used press from a broker, new rubber (rollers) would be expected. If you opt to purchase a press independently, budget new rollers for the entire machine.
With a two tower/true two color, each head is a true printing head. You would have to operate one to really appreciate the difference; words cannot do justice to the performance versus a T-Head.
The Ryobi 3302 is arguably the best small (A3/Tabloid) press. Hamada made a two tower small press, it may have been a fine machine; in the market there is also Toko, Chief, Multilith, Itek and AB Dick. I speak from experience to the 3302; it is common opinion they they are built like tanks. I work daily with two 3302's and a newer AB Dick 9810. For a days worth of single color work and envelopes, I won't fire up the 9810 until my arm is twisted. (A two color press is not able to back up (print the back side of) an uncoated sheet without smearing without several hours dry time.
All that said, I think your interest should steer towards, in this order:
Ryobi 3200/Itek 975 single color machine - (Itek/Ryobi were co-branded equivilant machines. My Ryobi notes are relevant to an Itek press as well.) Their small footprint and construction make them dependable. Envelope friendly. Parts and service have good availability.
AB Dick 9800 series single color - Envelope able, parts and service (?) readily avalable. Note - AB Dick presses use a 3 ply blanket (65 guage), Ryobi's use a 5 ply (98~102 guage). A lesser ply blanket is not as forgivable for envelope work. Either frequent replacement or more commonly, blanket savor (a swelling compound) is used for the smash correction.
Ryobi 3302/Itek 3985 - I am biased towards these machines; wouldn't be where I am withut one. The standard for a small shop. Their registration is necessary for mutli pass/3 & 4 color work, not pertinant for your position. Not as envelope friendly, but after 18 years with one, I don't hesitate an envelope changeover. If you look at one of these machines I know that my machine purchased in 1994 was more suited for envelopes; it has 5 feeder tapes; older machines had 7 which prohibit (?) their envelope feeding. I have a 7 tape machine and a 5 tape machine; I'd probably cut the two extra tapes without much consideration if the need ever arose.
AB Dick 360 - These are older than my myself and have had every operator attempt repairs and adjustments, some good, some bad. Get a 90 day warranty. I think they just barely print 11 x 17. You'll appreciate the small extra ability of everything else after some wear/tear and usage.
As important as everything above, for photo work/halftones you would benefit 100% with a continous dampening system on any press above. Your mention of large runs enforces that idea even more. Crestline or Kompac dampeners have gauranteed return on investment.
You have not mentioned plates - long conversation there...